*Print the N ^{th} Fibonacci number.*

Fibonacci numbers are defined by the recurring formula: *f _{n} = f_{n-1} + f_{n-2}*.

In Perl, you can assign two values at a time (see Task 48, *Swap two values*). You can use that technique for calculating the next Fibonacci number from the previous two. To bootstrap the algorithm, the two first values are needed. In one of the definitions of the Fibonacci row, the first two values are both 1 (sometimes, you may see that the first number is 0).

my ($a, $b) = (1, 1);

($a, $b) = ($b, $a + $b) for 3 .. 10;

say $b;

Another way of generating the sequence is to use the *sequence operator*. With this operator, you create a lazy list, which calculates its values according to the formula given in the generator argument.

my @fib = 1, 1, * + * ... *;

say @fib[9];

TheÂ `@fib`

array is a sequence. Its first two elements are both 1, but the rest are defined by the formulaÂ `* + *`

. This code creates an anonymous block that is equivalent to the body of a function with two arguments:Â `{$a + $b}`

. Thus, the element is a sum of its two neighbours. The right end of the sequence is specified asÂ `*`

, which means that it generates numbers infinitely, based on the demand.

The next line just takes the 10^{th} element of the array and prints its value:

55